The process control industry employs process variable transmitters to remotely monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter can provide an output related to the sensed process variable over a process control loop to a control room, such that the process can be monitored and controlled.
The process control loop can be a two-wire, 4-20 mA process control loop. With such a process control loop, the energization levels are low enough that even under fault conditions the loop generally will not contain enough electrical energy to generate a spark. This is particularly advantageous in flammable environments. Process variable transmitters can sometimes operate on such low energy levels that they can receive all electrical power from the 4-20 mA loop. The control loop may also have digital signals superimposed on the two-wire loop according to a process industry standard protocol such as the HART(copyright) digital protocol.
Low power Time Domain Reflectometry radar (LPTDRR) instruments have been used to measure the level of process products (either liquids or solids) in storage vessels. In Time Domain Reflectometry, electromagnetic energy is transmitted from a source, and reflected at a discontinuity. The travel time of the received pulse is based on the media through which it travels. One type of LPTDRR is known as Micropower Impulse Radar (MIR), which was developed by Lawrence Livermore National Laboratory. Since LPTDRR level transmitters typically determine level as a function of the time of travel of microwave signals to and from an interface or surface of the product, and since time of travel is dependent upon the dielectric constant of material through which the microwaves travel, it can be necessary to know the dielectric constant(s) ahead of time. This is particularly necessary when the storage tank contains multiple products layered on top of one another, thus creating multiple interfaces between products having different dielectric constants. Prior LPTDRR level transmitters have required that an operator of the transmitter enter a dielectric constant of the product in order to determine the level of the multiple interfaces. A method of determining the dielectric constant(s) of one or more products in a tank would be a significant improvement in the art.
Disclosed is a method and level transmitter which calculate a dielectric constant of a product in a tank. Low Power Time Domain Reflectometry Radar (LPTDRR) circuitry is controlled to calculate a time delay between transmission of microwave energy along a termination extending into the product in the tank and reflection of the microwave energy. In some embodiments, the dielectric constant of the product is calculated as a function of the time delay. In other embodiments, the dielectric constant is calculated by controlling the LPTDRR circuitry to calculate amplitudes of transmit and receive pulses. The dielectric of the product is calculated as a function of the amplitudes of the transmit and receive pulses.